Polyurethane materials containing ester-type linkages are particularly vulnerable to hydrolysis. The action known as hydrolysis results in cleavages of a portion of the chemical bonds of the polyurethane producing a polymer of low average molecular weight and/or reduced cross-linkage. This also results in liquification of the polymer, a process known as reversion. Reticulated polyurethane foam, which is used in aircraft fuel tanks to eliminate or attenuate explosions, is subject to this reversion action and therefore requires periodic removal and replacement. This is an expensive practice, particularly when the tanks are welded closed.
Water in the fuel, or from a humid environment in the ullage, acts on the exposed polyurethane foam in the fuel tank and cleaves the ester linkage. Some of the material resulting from the degradation of the polyurethane subsequently turns to a liquid which, when mixed with the hydrocarbon fuel, can cause engine damage.
The problem of polyurethane breakdown also occurs in polyurethane potting compounds used with electrical connectors. The potting compounds are used to seal the connectors to contacts in an insulator block. The surface of the potting compound is exposed to moisture and tends to react with the moisture and degrade.
This invention is suitable for use with polyester and polyether forms of polyurethanes. However, it is specifically applicable to the ester-type because this linkage is the most susceptible to hydrolysis and breakdown. The polyethers generally have greater inherent stability.
The present invention involves a chemical process whereby a diisocyanate or polyfunctional epoxy compound is reacted with the polyurethane and introduced into the structure of the molecule. The additive is a cross-linking type material and may serve to reconnect the portion of the molecule cleaved during hydrolysis. The principal advantage of the process is that the useful properties of the polyurethane foam material are restored or modified in situ.
By means of this process partially hydrolyzed or reverted foam can be reconstituted in the fuel tank, without the difficult and time consuming removal of the foam heretofore necessary.
In addition, this process allows the chemical properties of the foam to be altered and tailored after it is manufactured and prior to its installation. That is, the process can be used to restructure the polyurethane material after is is made into reticulated form.
This process allows the physical properties of the foam to be altered and tailored to improve the hydrolytic stability at any convenient time during the life time of the foam. For example, the treatment to reconstitute the foam can be scheduled as a routine operation to be carried out during a regular aircraft overhaul and repair period.
These and other objects and advantages will become apparent hereinafter.